Fig. 3. Power spectra calculated using the discrete FT from raw ECG traces containing either 128 consecutive heart beats (black spectra) or ventilation cycles (grey spectra). All plots are from the same individual. (A) Spectra generated when _O2 (1.8 mmol O₂ kg^–1 h^–1), f_H (49 min^–1) and f_V (47 min^–1) were high due to surgery and placement in a respirometer; the peaks are very narrow, showing very little variability in either f_H or f_V. (B) After a period of no disturbance and starvation, _O2 (0.91 mmol O₂ kg^–1 h^–1), f_H (25 min^–1) and f_V (28 min^–1) were reduced dramatically and the f_H spectral peaks were spread within the bandwidth. (C) Ingestion of a 5%b.m. ration resulted in a very high _O2 (2.11 mmol O₂ kg^–1 h^–1), f_H (43 min^–1) and f_V (47 min^–1); the spectral peaks reduced in amplitude but remained with a broad bandwidth. (D) Bilateral vagotomy resulted in a moderately high _O2 (1.41 mmol O₂ kg^–1 h^–1), f_H (53 min^–1) and f_V (53 min^–1), and the spectral peaks were very narrow, indicating little variability.